Concepts Introduced in Chapter 7

Transcription

1 Concepts Introduced in Chapter 7 Storage Allocation Strategies Static Stack Heap Activation Records Access to Nonlocal Names Access links followed by Fig. 7.1 EECS 665 Compiler Construction 1

2 Activation Records An activation record is usually a contiguous block of storage that holds information relevant to one activation (execution) of a routine. Activation records can be placed in different storage areas depending on the run-time environment of the programming language. Static data: FORTRAN Stack: ALGOL, C, Pascal, Ada, C++ Heap: Functional Languages LISP, ML, Haskell, Erlang followed by Fig. 7.2, 7.3, 7.4 EECS 665 Compiler Construction 2

3 Call Graphs Call graphs represent the possible sequence of function calls in a program. followed by Fig. 7.5 EECS 665 Compiler Construction 3

4 Typical Actions During Call / Return Calling sequence actions caller assigns the actual arguments. caller stores return address. caller jumps to callee. callee adjusts stack pointer for the activation record. callee saves nonscratch register values it will use. Return sequence actions callee assigns the return value. callee restores nonscratch register values and stack pointer address. callee jumps to the return address. caller accesses the return value. EECS 665 Compiler Construction 4

5 Storage Allocation Strategies Static Allocation - lays out storage for all data objects at compile time Restrictions size of object must be known and alignment requirements must be known at compile-time no recursion no dynamic data structures EECS 665 Compiler Construction 5

6 Storage Allocation Strategies (cont.) Stack allocation - manages run-time storage as a stack (LIFO model) Activation record is pushed on as function is entered. Activation record is popped off as function is exited. Restrictions Values of locals cannot be retained when an activation ends. A called activation cannot outlive a caller. followed by Fig. 7.6, 7.7 EECS 665 Compiler Construction 6

7 Issues to Address in Calling Conventions Responsibility of the caller versus the callee. Identifying registers for special purposes. stack pointer, frame pointer, return address Preserving the value of registers across calls. callee save, caller save Passing arguments. through registers, on the stack followed by Fig. 7.8 EECS 665 Compiler Construction 7

8 Dangling Reference main ( ) { int *p; p = dangle ( ); } int *dangle ( ) { int i = 23; return &i; } EECS 665 Compiler Construction 8

9 Storage Allocation Strategies (cont.) Heap Allocation - allocates and deallocates storage as needed at run-time from a data area known as a heap Most flexible Most inefficient EECS 665 Compiler Construction 9

10 Heap Storage Reclamation Strategies explicit reclamation used in Pascal, Ada, C, C++ complicates programming implicit reclamation used in Lisp and Java reference counts mark and sweep (garbage collection) EECS 665 Compiler Construction 10

11 Access to Nonlocal Names The scope of a declaration in a block-structured language is given by the most closely nested rule: The scope of a declaration in a block B includes B. If a name X is not declared in a block B, then an occurrence of X in B is in the scope of a declaration of X in an enclosing block B such that B has a declaration of X, and B is more closely nested around B than any other block with a declaration of X. EECS 665 Compiler Construction 11

12 Lexical Scope without Nested Procedures The lexical-scope rules for a language without nested procedures, such as C, are simpler than those of a block-structured language, such as Pascal. The scope of a declaration outside a function consists of the function bodies that follow the declaration. EECS 665 Compiler Construction 12

13 Lexical Scope without Nested Procedures (cont.) C functions accesing a nonlocal variable a. int a[11]; readarray() {... a... } int partition(int y, int z) {... a... } quicksort(int m, int n) {... a... } main() {... a... } Note that all functions access the same a. Functions in C can be easily passed as parameters and can be returned as a function result. EECS 665 Compiler Construction 13

14 Lexical Scope with Nested Procedures In a block structured language, a procedure can access nonlocal names that are local variables in other procedures. Since such variables are local variables in other procedures, they are placed in activation records on the run-time stack. How can they be accessed at run-time? followed by Fig EECS 665 Compiler Construction 14

15 Referencing Variables with Access Links An access link points to most recent activation of the procedure that contains the current procedure. Suppose Np is the nesting depth of procedure p that refers to a nonlocal variable a. Na is the nesting depth of procedure that contains a. Np - Na access links would have to be traversed when in procedure p to get to the activation record that contains a. followed by Fig. 7.11, 7.12, 7.13 EECS 665 Compiler Construction 15

16 Establishing Access Links Suppose p calls x. Two cases for setting up the access link. If Np < Nx Procedure x is nested more deeply than procedure p. x must be declared within p. The access link should point to the caller. If Np Nx Follow Np Nx + 1 access links from the caller to reach the activation record of the procedure that encloses the called procedure x most closely. followed by Fig EECS 665 Compiler Construction 16